(a) Field of the Invention
The present invention relates to a copolymer having a substantially syndiotactic structure of propylene, a method for preparing it, and its use. More specifically, the present invention relates to a copolymer of propylene which is prepared by graft polymerizing a radical polymerizable unsaturated compound on a syndiotactic propylene homopolymer having a relatively high tacticity or a syndiotactic copolymer having a relatively high tacticity of propylene and a small amount of another olefin, a method for preparing the above-mentioned copolymer, and its use.
(b) Description of the Prior Art
An isotactic polypropylene is inexpensive and relatively excellent in the balance of physical properties, and thus it is utilized in various uses. On the other hand, a syndiotactic polypropylene has been known for a long period of time. However, such a syndiotactic polypropylene is prepared by polymerization at a low temperature in the presence of a conventional catalyst comprising a vanadium compound, an ether and organic aluminum, and the syndiotactic polypropylene prepared by this process is poor in syndiotacticity and has elastomer-like characteristics. Therefore, it is scarcely considered that the thus prepared syndiotactic polypropylene exerts its inherent characteristics.
On the contrary, a polypropylene having good tacticity, i.e., a syndiotactic pentad fraction of more than 0.7 has been discovered for the first time by J. A. Ewen et al. which can be obtained by the use of a catalyst comprising a transition metal compound having asymmetric ligands and an aluminoxane (J. Am. Chem. Soc., Vol. 110, 6255-6256, 1988).
A syndiotactic polypropylene or a copolymer having a substantially syndiotactic structure of propylene and another olefin which is obtained by the above-mentioned method is relatively excellent in physical properties, but this kind of polypropylene or copolymer has a bad affinity for a polymer containing another polar group or a metal, i.e., it is poor in adhesive properties similarly to the isotactic polypropylene.
On the other hand, a polymer prepared by graft polymerizing a monomer containing a carboxyl group such as maleic anhydride on the isotactic polypropylene is utilized for the purpose of improving adhesive properties to another polyolefin or coating properties. However, this techique has problems that the radical decomposition of the polypropylene which is the base polymer occurs in a manufacturing step, with the result that the molecular weight of the resultant copolymer noticeably decreases, and that when manufactured in a liquid phase, it is difficult to separate the copolymer. On the contrary, if a product containing a syndiotactic polypropylene as a base is prepared, physical properties which have not been heretofore present can be expected.